Why it does not follow law of conservation of mass? 12g of C is heated in vacuum and on cooling there is no change in mass

Q. Which of the following equations does not follow the law of conservation of mass?

Q. Which of the following equations does not obey the law of conservation of mass?

Q.

In one complete cycle, a heat engine extracts heat $Q_h$ from a thermal reservoir, does work W and does not eject any heat into the environment. This heat engine is impossible because its operation violates:

Q. I understood that momentum is defined as product of mass and velocity
But what is the meaning of 'change in momentum'?
Like it made no sense to me.. How can momentum change once the body is in motion?
Does the mass and velocity change or does there occur a simple change in motion?
If it is change in motion then why not term it change in motion instead of change in momentum?
Please explain this concept of 'change in momentum' to me im detail.

Q. When water is heated at its boiling point or melting point, its temperature does not change.
a) What is meant by boiling point and melting point?
b) What are the boiling and freezing points of water?
c) Why is there no change in temperature?